Method for producing a master alloy for use in aluminum casting processes

ABSTRACT

A method for producing a master alloy for use in aluminum casting processes in which an aluminum melt containing 0.02-6 percent by weight titanium and 0.01-2 percent by weight boron is produced under conditions under which the boron is bound to titanium in the form of titanium diboride, whereafter the melt containing titanium diboride is held under agitation at a temperature ranging from the melting point of the material to 900* C for a period of at least 15 minutes and at most 9 hours.

United States Patent [1 1 Backerud Jan. 15, 1974 METHOD FOR PRODUCING AMASTER ALLOY FOR USE IN ALUMINUM CASTING PROCESSES [75] Inventor: StigLennart Backerud, Akersberga,

Sweden [73] Assignee: Granges Aluminium AB,

Kubikenborg, Fack, Sundsvall, Sweden [22] Filed: Apr. 27, 1971 [21]Appl. No.: 137,986

[30] Foreign Application Priority Data Apr. 28, 1970 Sweden 5881/70 [52]US. Cl. 75/138, 75/68 R [5 l] Int. Cl C22c 1/02 [58] Field of Search75/138, 135, 68 R [56] References Cited UNITED STATES PATENTS 3,464,8169/1969 Biddulph 75/1 38 2,578,098 l2/l95l Southard 75/138 PrimaryExaminer-Richard 0. Dean AttorneyWaters, Roditi, Schwartz & Nissen [57]ABSTRACT 9 Claims, No Drawings METHOD FOR PRODUCING A MASTER ALLOY FORUSE IN ALUMINUM CASTING PROCESSES The present invention relates to amethod for producing a master alloy which can be added to an aluminummelt before the melt solidifies, thereby to obtain finer grain size ofthe cast aluminum product with subsequent increase in the quality of thesame.

It is known that in order to obtain a satisfactory product from analuminum casting process, for example, it is necessary to add asubstance which facilitates the formation of crystals during the periodof solidification, the substance preventing the aluminum melt fromsolidifying to a coarse-crystal product. To this 'end, different grainrefining substances are generally incorporated in the aluminum as masteralloys, which are added to the aluminum melt in solid form, for examplein the form of small ingots or a wire which is continuously fed into themelt. The master alloy may also be added in a molten state.

The master alloys previously used have comprised mainly titanium, boronand a combination of titanium and boron. Typical master alloys contain2-l0 percent by weight titanium in aluminum, 0.3-5 percent by weightboron in aluminum and 2-10 percent by weight titanium together with0.3-5 percent by weight boron in aluminum. A usual composition is onecontaining 5 percent by weight titanium and 1 percent by weight boron inaluminum. Master alloys of this type are available commercially.

lt has now been surprisingly found that by producing atitanium-boron-aluminum master alloy in a special manner it is possibleto improve considerably the grain refinement while using considerablylower total quantities of titanium and boron than has been possible withthe hitherto commercially available master alloys.

Master alloys containing titanium and boron are normally produced bydissolving the required quantities of titanium and boron in an aluminummelt at temperatures in excess of approximately L200 C. When practicingthis method, it is first necessary to dissolve a specific quantity oftitanium before adding the boron. The boron is added in the form of aboron salt, normally potassium borofluoride (KBF The boron salt isdissociated in the melt and the liberated boron then rapidly combineswith the titanium present in the melt. It is also possible to dispersetine-grain titanium diboride in the melt. In accordance with the presentinvention there is produced a master alloy which is intended to be addedto an aluminum melt to afford a grain refining effect during thesolidification period, an aluminum melt containing 0.02-6 percent byweight titanium and 0.0l-2 percent by weight boron being produced, inwhich the boron is bound to the titanium in the form of titaniumdiboride, by either first dissolving titanium at a temperature such thatthe quantity added passes into solution, and then adding boron, or bydispersing titanium boride in an aluminum melt, and the method ischaracterized by the step of maintaining the melt containing titaniumdiboride at a temperature between the melting point of the mixture and900 C while stirring the melt and for a period of time of at least minutes and at most 9 hours.

If large quantities of titanium, for example of the order of 10 percentby weight, are to be dissolved, it is necessary, for thermodynamicreasons, that the temperature during the dissolution phase reaches atleast 1,200 C. Consequently, it is naturally necessary to cool thealuminum melt rapidly down to a temperature below 900 C, in order toprevent the occurrence of undesirable reactions. Since it is difficultto rapidly cool the alloy to a temperature immediately above the meltingpoint a particularly suitable method for carrying out such a coolingprocess is one in which alloy is cast in small, water-cooled moulds,whereafter the metal is remelted at a temperature below 900 C.

The titanium content of the master alloy is preferably 0.2-2 percent byweight and the boron content is preferably.0.l-l percent by weight andthe temperature used during the dissolution phase is from l,200 to l,500C. The alloy is then cooled to the holding temperature between themelting point and 900 C. A preferred holding temperature is 680-720 Cand apreferred holding time is from 45 minutes to 2.5 hours. Subsequentto the heat treatment process, the pre-alloy can be used directly orsubsequent to solidifying, although it is normal practice to decant themolten master alloy to prevent the formation of large agglomerates oftitanium diboride and other impurities from accompanying the masteralloy.

The conditions prevailing in the aluminum-titanium system are evidentfrom available constitutional diagrams, from which it can be seen thatpure aluminum solidifies at approximately 660 C and that a peritecticsolidification line exists from a titanium content of roughly 0.5percent by weight at 665 C to the stoichiometric composition for Al Tiat roughly 37.5 percent by weight Ti. In order that Al Ti can be formed,the content of Ti at 665 C must thus be at least 0.15 percent by weight.At 900 C the liquid solubility for titanium equals 1 percent.

When titanium and boron are dissolved in aluminum, a compound betweentitanium and boron, TiB is rapidly formed, Al Ti crystallizing out atreduced temperature during the holding period to embrace the compound.The formation of Al Ti presumes that the concentration of Ti in thesystem exceeds the content necessary for forming Al Ti at thetemperatures in question. In this particular instance, a titaniumconcentration gradient is obtained around the TiB grains. Thisconcentration gradient is obtained as a result of the fact that titaniumis disassociated from titanium diboride and is replaced therein withaluminum. This enables the titanium diboride and aluminum diboride tohave the same crystal structure and to replace each other in the crystallattice.

It is thus necessary to exceed the solubility limit or liquiduscurve inthe constitutional diagram for Al Ti, which can be effected by raisingthe concentration of titanium or by changing the position of thesolubility curve by means of appropriate additives. In this way, Al Tiwill crystallize around the TiB grains and form small crystals, whichconstitute the actual crystallization nuclei. The formation of Al Titakes place during the holding time at the aforementioned temperatureinterval of the invention. If the titanium content of the master alloyis of such magnitude that Al Ti can be formed in the whole melt, largequantities of Al Ti crystals will be formed, which when the master alloyis used will dissolve and give high titanium contents to the finalproduct, but will of course also act as crystallization nuclei to alesser extent, owing to the fact that these crystals will becomeconsiderably larger and fewer than those which are formed around the TiB-grains.

The initally irregular grains of TiB will, after approximately 1 hour,have been embraced by a more regularly shaped crystal shell comprisingsubstantially Al Ti. The formed crystals added to the aluminum melt areable to refine the grains rapidly and effectively. If the master alloyis not subjected to the crystallization of Al Ti around the TiB-particles during the holding period and during simultaneous agitationof the system, TiB will form aggregates which will be practicallytotally precipitated out by gravitational separation, and will eithernot be included during the casting process or will be entrained with thecasting material, thereby rendering its use impossible for, for example,foil rolling, where the agglomerated TiB -particles cause the foil to betorn during the rolling operation. For the same reason, a large quantityof TiB will fall to the bottom of the furnace without fulfilling itsfunction as a grain refining agent, whereupon it becomes necessary toadd the TiB in excessive quantities, which considerably impairs theeconomy of aluminum casting processes using this agent. A large additionof grain refining agent also causes a large quantity of titanium to bedissolved in the melt. An increase in the titanium content of aluminumgives rise to several undesirable effects, such as the formation offeathery grains and changes in the conductivity of the final product,

The new master alloy of the present invention can be used inconsiderably small quantities or with a lower content of titanium andboron, since it is possible to utilize actively all the titanium andboron present therein.

it is obvious that the final, desired aluminum melt can be consideredtotally as a master alloy and that the melt can be treated in a mannerwhereby titanium and boron are first dissolved at higher temperaturesand the whole melt than maintained at a temperature of approximately 700C for a period of 1 hour under agitation. in this way grain refinementwould be equivalent to that obtained with the master alloy of thepresent invention. However, such treatment of an aluminum melt isexpensive, extremely difficult to carry out technically and gives anundesirable content of titanium in the product. It is, instead,particularly desirable to produce a master alloy which can be used incontinuous casting processes externally of the furnace in a specialcontainer or in the actual pouring stream. The master alloy of thepresent invention is particularly suited for this purpose, since it canbe passed to the melt just before the melt is to be transferred to themould and intimately blended with the melt. In this way, the grainrefining agent is able to exert its influence immediately and a superiorproduct is obtained with a considerably smaller total quantity oftitanium and boron in the finished product.

What I claim is:

l. A method for preparing a master alloy intended to be added to analuminum melt for refining the grains of the aluminum duringsolidification thereof comprising the steps of a. preparing an alloymelt consisting essentially of 0.02-6 percent by weight of titanium and0.0l-2 percent by weight of boron, the balance being aluminum andwherein the boron is bound to titanium in the form of titanium diboride,and

b. maintaining the alloy melt at a temperature between the melting pointof the alloy and 900 C. for a period of time of at least 15 minutes andat most 9 hours with agitation until the grains of titanium diboride inthe alloy become embraced by a crystal shell comprising substantially AlTi.

2. A method according to claim 1 wherein the cooling step comprisescooling the alloy melt to a temperature below the melting point of thealloy and then reheating the cooled alloy to a temperature between themelting point of said alloy and 900 C.

3. A method according to claim 2 wherein the step of cooling the alloymelt to a temperature below the melting point of the alloy comprisescasting the alloy melt into cooled moulds.

4. A method according to claim 1, wherein the titanium is present in anamount of 002-2 percent by weight, based on the total weight of thealloy.

5. A method according to claim 1, wherein the boron is present in anamount of 0.0l-l percent by weight, based on the total weight of thealloy.

6. A method according to claim 1, wherein the alloy melt is maintainedat a temperature of 680720 C.

7. A method according to claim 6, wherein the alloy melt is maintainedat 680-720 C for from 45 minutes to 2.5 hours.

8. A method according to claim 1 wherein the step of preparing the alloymelt comprises dispersing titanium diboride in molten aluminum.

9. A method according to claim 1 wherein the step of preparing the alloymelt comprises adding titanium and boron to molten aluminum at atemperature above about l,200 C.

2. A method according to claim 1 wherein the cooling step comprisescooling the alloy melt to a temperature below the melting point of thealloy and then re-heating the cooled alloy to a temperature between themelting point of said alloy and 900* C.
 3. A method according to claim 2wherein the step of cooling the alloy melt to a temperature below themelting point of the alloy comprises casting the alloy melt into cooledmoulds.
 4. A method according to claim 1, wherein the titanium ispresent in an amount of 0.02-2 percent by weight, based on the totalweight of the alloy.
 5. A method according to claim 1, wherein the boronis present in an amount of 0.01-1 percent by weight, based on the totalweight of the alloy.
 6. A method according to claim 1, wherein the alloymelt is maintained at a temperature of 680*-720* C.
 7. A methodaccording to claim 6, wherein the alloy melt is maintained at 680*-720*C for from 45 minutes to 2.5 hours.
 8. A method according to claim 1wherein the step of preparing the alloy melt comprises dispersingtitanium diboride in molten aluminum.
 9. A method according to claim 1wherein the step of preparing the alloy melt comprises adding titaniumand boron to molten aluminum at a temperature above about 1,200* C.